Controlled release of chemical admixtures

ABSTRACT

A controlled release formulation for a cement-based composition can be produced by intercalating an admixture (e.g. an accelerator, a set retarder, a superplasticizer) for the cement-based composition into a layered inorganic material (e.g. a layered double hydroxide (LDH)). A cement-based composition containing a cement-based material (e.g. cement, mortar or concrete) and such a controlled release formulation has better workability, especially in respect of slump-loss characteristics. With such a formulation release of an admixture in a cement-based composition may be controlled.

CROSS-REFERENCE APPLICATIONS

This application claims the benefit of United States patent applicationU.S. Ser. No. 60/703,895 filed Aug. 1, 2005.

FIELD OF THE INVENTION

The present invention is related to control of admixture release incompositions, especially in cement-based compositions.

BACKGROUND OF THE INVENTION

Control of admixture action in concrete and other materials is limitedby the methods and timing of delivery. Admixtures are most often addedat time of mixing, which is not necessarily optimal for the desiredchemical effects. For instance, it is sometime desirable to delayrelease of compounds such as superplasticizers, retarders, accelerators,and other additives.

The prior art describes “encapsulation” procedures for delivery ofchemicals. Such procedures often rely on mechanisms involvingdissolution (coating), diffusion (membranes), desorption (porousmaterials), and mechanical dispersion (during mixing), which areexpensive and time-consuming.

Pronounced anion exchange capacity of LDHs and LDH-like materials makesinterlamellar ion exchange by organic and inorganic anions versatile andeasy. LDHs have been investigated extensively in a wide range ofapplications such as catalysts, ceramic precursors, adsorbents,bio-organic nanohybrids, and also as scavengers of pollutant metals andanions. Recent research has shown great flexibility of the anionic claysin tailoring chemical and physical properties of materials to be usedfor specific application, e.g. molecular recognition, optical storage,batteries, etc. Furthermore, by introducing various transition and noblemetals into the sheets of the LDH structure, researchers have been ableto produce catalyst precursors. More recently, there have been atremendous number of new developments using a LDH as a matrix forstorage and delivery of biomedical molecules and as a gene carrier.

Development of new inexpensive materials for programmed delivery andaction control of admixtures in cement-based compositions would presenta significant technological advance.

SUMMARY OF THE INVENTION

In one aspect of the invention, there is provided a controlled releaseformulation for a cement-based composition comprising a layeredinorganic material and an admixture for the cement-based composition.

In another aspect of the invention, there is provided a cement-basedcomposition comprising: a cement-based material; and, a controlledrelease formulation comprising a layered inorganic material and anadmixture for the cement-based composition.

In yet another aspect of the invention, there is provided a method forcontrolling release of an admixture in a cement-based compositioncomprising intercalating the admixture into a layered inorganic materialto form a controlled release formulation and adding the controlledrelease formulation to a cement-based material.

The present invention provides a nano-technology approach to controldelivery of admixtures in cement-based compositions. De-intercalation orrelease of the admixture can be actively programmed through controlledchemistry involving, for example, type of layered inorganic material,charge density, concentration, and/or pH. The admixture may be anorganic or an inorganic species. Layered double hydroxides (LDHs) inwhich organic admixtures are intercalated are particularly efficacious.

The present invention allows in situ real-time delivery of admixtures tocement-based compositions. The performance of admixtures andcement-based products containing them can be enhanced, for example,improved modulation of slump loss of cement-based compositions may beachieved. The present invention thus provides an effective andcontrollable method of releasing admixtures into cement-basedcompositions.

Layered inorganic materials preferably comprise naturally occurringhydrotalcite (Mg₆Al₂(OH)₁₆CO₃.4H₂O), a layered double hydroxide (LDH),or a mixture thereof. The layered inorganic material preferablycomprises a layered double hydroxide or mixture thereof. The layereddouble hydroxide preferably comprises a synthetic LDH or mixturethereof.

LDHs are structurally related to brucite, Mg(OH)₂, the same way AFmphases are to Portlandite. Structurally, a layered double hydroxide(LDH) is similar to brucite, Mg(OH)₂, with the replacement of some Mg²⁺cations by trivalent ions. Excess positive charge is neutralized byinterlayer anions.

LDHs are preferably compounds of formula (I):[M²⁺ _(1−x)M³⁺ _(x)(OH)₂]^(x+)[A^(n−) _(x/n) .mH₂O]^(x−)  (I)where M²⁺ is a divalent metal cation, M³⁺ is a trivalent metal cation,A^(n−) is an anion, x is a number from 0 to 1 but not 0, n is a number 1or greater, and m is a number 0 or greater.

M²⁺ and M³⁺ are such that their ionic radii can be accommodated withinan octahedral configuration of OH groups. M²⁺ is preferably Ni²⁺, Zn²⁺,Mn²⁺, Ca²⁺, or a mixture thereof. M³⁺ is preferably Al³⁺, Ga³⁺, Fe³⁺,Cr³⁺, or a mixture thereof. The number x is a molar ratio defined byM³⁺/(M²⁺+M³⁺), and preferably is in a range of from about 0.1 to 0.5,which corresponds to a layer charge density of from about 2 to 4positive charges per nm². More preferably, 0.2≦x≦0.33. Particularlypreferred values of x are 0.25 and 0.33, corresponding to chargedensities of 33.5 Å/charge and 25 Å/charge, respectively.

A^(n−) is an anion that compensates for excess positive charge inducedby M³⁺ substitution of M²⁺. A^(n−) may be small or bulk organic orinorganic molecules or layered materials. Preferably, A^(n−) is aninorganic anion, more preferably NO₃ ⁻, Cl⁻, CO₃ ²⁻, SO₄ ²⁻ or a mixturethereof. The number n is preferably a number from 1 to 4.

The number m defines an amount of water localized in the interlamellarspace of the LDH, and depends on size and charge of the anion A^(n−), onrelative humidity and on x.

An LDH may be built up from brucite, Mg(OH)₂, by isomorphoussubstitution of divalent cations. Mg²⁺ ions may be replaced by M³⁺ ionsand/or other M²⁺ ions. Replacement of Mg²⁺ by M³⁺ ions generates anexcess of positive charge within the inorganic layers, which has to bebalanced by incorporation in the interlayer space of anions, i.e.A^(n−). In addition to anions, the interlayer space region can alsocontain water molecules connected to the inorganic layers via hydrogenbonding.

LDHs may also be prepared by co-precipitation methods using M²⁺ and M³⁺sources at constant pH under basic conditions. In co-precipitationmethods, the pH is generally higher than or equal to the pH at which themore soluble metal hydroxide precipitates. Preferably, the pH is in arange of from about 8 to 10. Their high anion exchange capacity (2.4 to4.1 milliequivalents per gram) makes the interlamellar ion exchange byorganic and inorganic anions versatile and easily achieved.

The admixture for cement-based composition may be one or more organicand/or inorganic molecules, and may comprise small molecules, polymersor mixtures thereof. Preferably, the admixture comprises an acceleratorfor reducing set time, a retarder for delaying set time, asuperplasticizer, an air-entraining agent for freeze-thaw resistance, acorrosion inhibitor, an expansive admixture for minimizing shrinkage, ashrinkage reducing admixture, a water repelling admixture, a waterreducer (including high-range water reducers), an alkali-aggregatereaction inhibitor (e.g. lithium-based salts), or a mixture thereof.Preferably, the admixture comprises an organic molecule.

Controlled release formulations may be prepared by intercalating anadmixture in the layered inorganic material, for example by anionexchange. This method has been widely explored and used in fields of LDHapplications such as catalysis, optical materials, separation science,and medicine. LDHs have greater anion exchange capacities (about 2.4 to4.1 milliequivalents per gram) compared with cation exchange capacities(about 0.7 to 1.0 milliequivalents per gram) in conventional clays suchas montmorillonite. The anion exchange sequence in LDHs is as follows:CO₃ ²⁻>>SO₄ ²⁻>>OH⁻>F⁻>Cl⁻>Br⁻>NO₃ ⁻>I⁻. Layered hybridorganic-inorganic materials may be obtained via an ion exchange reactionof interlayer ions, preferably nitrates, of the layered host materialswith guest anions. Concentration of the admixture in the layeredinorganic material has an impact on performance of the formulation.Preferably, the concentration is within a range of about 0.05-1 M, forexample in a range of about 0.5-0.7 M (e.g. 0.5 M and 0.7 M).

Cement-based materials include cement, mortar, concrete, etc. Cement isgenerally formulated by mixing water with dry powder cementitiousmaterial. Mortar is generally formulated by mixing water with dry powdercementitious material and fine aggregate (e.g. sand). Concrete isgenerally formulated by mixing water with dry powder cementitiousmaterial and both fine aggregate (e.g. sand) and coarse aggregate (e.g.stone). Cementitious material includes, for example, Portland cement,high alumina cement, magnesium phosphate cement, gypsum, etc., ormixtures thereof. Other inorganic chemical admixtures may be present inthe cement-based composition, for example, pozzolanic materials (e.g.fly ash, slag, silica fume, lime, etc., or mixtures thereof) tosupplement or replace cementitious materials.

Cement-based compositions may be produced by adding, preferably withmixing, a controlled release formulation of the present invention to acement-based material. The controlled release formulation may be addedtogether with a cementitious material (and aggregate if desired) towater, preferably with mixing, to form the cement-based composition.Time and method of introduction of the controlled release formulation inthe cement-based composition has an impact on performance. Preferably,the controlled release formulation is first added to the cementitiousmaterial to form a mixture, followed by addition of water to themixture. Water-cement ratio (w/c) of the cement-based material ispreferably about 0.1-10, for example 0.3-0.5 (e.g. 0.3, 0.4 or 0.5). Thecontrolled release formulation is preferably present in the cement-basedcomposition in an amount from about 0.2% to about 10%, more preferablyabout 1% to about 5%, for example about 3.5% to about 3.7%, based onmass of cement.

Release of molecules from the interlamellar space of an LDH depends onacid-base reaction pH values. Since cement pore solution is a basicmedium, pH of the cement-based material is preferably in a range of12-14. At lower pH values release of molecules is slower.

Further features of the invention will be described or will becomeapparent in the course of the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be more clearly understood, embodimentsthereof will now be described in detail by way of example, withreference to the accompanying drawings, in which:

FIG. 1 depicts X-ray diffraction (XRD) patterns for a synthetic CaAl LDHand for synthetic CaAl LDH having organic molecules intercalatedtherein;

FIG. 2 depicts a schematic diagram of organic molecules intercalated insynthetic LDHs;

FIG. 3 depicts X-ray diffraction patterns for CaAl LDH and for CaAl LDHhaving Disal™ intercalated therein;

FIG. 4 depicts a TGA/DTG (thermogravimetric analysis/derivativethermogravimetry) curve for CaAl LDH;

FIG. 5 depicts FT-IR spectra of CaAl LDH, CaAlNBA LDH, CaAl26NS LDH, andCaAl2NS LDH;

FIGS. 6 a and 6 b depict scanning electron micrographs (SEM) of CaAl LDHand CaAlNBA LDH;

FIG. 7 depicts calorimetry curves for Disal™ and Calcium AluminateDisal™ (CADisal);

FIGS. 8 a and 8 b depict X-ray diffraction profiles of twodeintercalation processes of nitrobenzoic acid (NBA);

FIG. 9 depicts a plot of slump loss of cement pastes produced withDisal™ alone and with a controlled release formulation (CADisal) of thepresent invention as a function of mixing time at room temperature;

FIG. 10 depicts a plot of slump loss of mortars produced with Disal™alone and with a controlled release formulation (CADisal) of the presentinvention as a function of mixing time at room temperature; and,

FIG. 11 depicts a plot of slump loss of concretes produced with Disal™alone and with a controlled release formulation (CADisal) of the presentinvention as a function of mixing time at room temperature.

DESCRIPTION OF PREFERRED EMBODIMENTS Examples

In the Examples, a series of controlled release formulations have beendeveloped and tested using a number of analytical and engineeringtechniques. X-ray diffraction (XRD), thermogravimetric analysis (TGA),infrared spectroscopy (IR), scanning electron microscopy (SEM), X-rayfluorescence (XRF), and nuclear magnetic resonance (NMR) were used tocharacterize the controlled release formulations. Conduction calorimetry(CC) was used to study the effect of controlled release formulations onthe hydration kinetics of cement phases. Rheological properties/changesof paste, mortar and concrete were studied by analysing theeffectiveness of the controlled release formulations in controlling theslump-loss versus time characteristic.

Materials:

All reactions were carried out using reagent grade chemicals (>98%purity) obtained from Sigma-Aldrich and Fisher Scientific withoutfurther purification. Deionized distilled water was used for thepreparation of aqueous solutions.

Organic admixtures were: ortho, para and meta nitrobenzoic acid salts;ortho, para and meta aminobenzoic acid salts;naphthalene-2,6-disulfonate; naphthalene-2-sulfonate; salicylic acidsalt; citric acid salt; benzene sulfonic acid salt; polyacrylic acidsalts, polyvinyl alcohol; and Disal™ (a poly naphthalene sulfonatesodium salt superplasticizer).

Divalent metal (M²⁺) salts including Ca(NO₃)₂.4H₂O and Zn(NO₃)₂.6H₂O,and trivalent metal (M³⁺) salts including Al(NO₃)₃.9H₂O andFe(NO₃)₃.9H₂O were used in the presence of sodium hydroxide (NaOH) andsodium nitrate (NaNO₃) at M²⁺/M³⁺=2 and 3.

Development of controlled release formulations was achieved in twosteps: synthesis of an M²⁺-M³⁺-based LDH material with exchangeableanions such as nitrates, hydroxides, chlorides, etc.; and, intercalationof organic molecules within the LDH material by anion exchange. Atypical procedure is described in Example 1.

Example 1 Preparation of Controlled Release Formulations

Preparation of a CaAl LDH

A CaAl LDH was prepared by a pH controlled co-precipitation technique ofthe corresponding metal nitrate salts at room temperature. Typically, asolution containing 0.28 moles of Ca(NO₃)₂.4H₂O and 0.12 moles ofAl(NO₃)₃.9H₂O in 320 ml of distilled water was added drop wise to asolution containing 0.6 moles of NaOH and 0.4 moles of NaNO₃. The pH ofthe final mixture was 9.6. The suspension was heated for 16 h at 65° C.with vigorous stirring, after which the solid precipitate was collectedby filtration and washed thoroughly with distilled water several timesand then with acetone. The cake-like material was then dried 16 h at100° C. in vacuum.

Intercalation of an Organic Molecule in CaAl LDH

Intercalation reactions of organic molecules were performed using anionexchange reactions known in the art. The reactions were carried outunder nitrogen to avoid contamination with carbonates from theatmosphere. CaAlNBA LDH, CaAl26NS LDH, and CaAl2NS LDH were obtained byanion exchange of nitrates with nitrobenzoic acid (NBA),naphthalene-2,6-disulfonic acid (26NS), and naphthalene-2-sulfonic acid(2NS) salts, respectively. Typically, 2.5 g of the CaAl LDH compoundwere dispersed in 250 ml 0.1 M aqueous solutions of organic salts. Themixtures were allowed to react for 16 h, under nitrogen and vigorousstirring at 65-70° C. The controlled release formulations were isolatedby filtration and washed thoroughly with distilled water and acetoneseveral times. They were then dried under vacuum for 4 hours at 100° C.A similar method may be used to produce CADisal, which is Disal™intercalated in a CaAl LDH. Disal™ is a commercial superplasticizer.

Example 2 Characterization of Controlled Release Formulations

Powder X-ray diffraction (XRD) was performed on a Scintag XDS 2000 X-Raydiffractometer using Cu—Kα radiation at 45 kV and 35 mA between 4 and650 (2θ with a graphite secondary monochromator). FTIR spectra wererecorded on a Bohmem MB 100 instrument. Thermal analyses on powdersamples (10-20 mg) were carried out using a Seiko simultaneous thermalanalyzer (STA) TG/DTA320 in flowing Ultra zero air (150 mL/min) at 20°C./min from room temperature to 1000° C. Scanning electron microscopyand chemical analyses (SEM/EDX) were conducted using a Cambridge SystemsStereoscan™ 250 instrument equipped with an Oxford Instruments Inca™ 200EDS.

X-ray Diffraction (XRD) Analysis

Ca/Al (molar) ratio found for the CaAl LDH by EDS (energy dispersivespectroscopy) was equal to 2, suggesting the following formula:Ca₂Al(OH)₆NO₃.mH₂O. The diffraction pattern for the synthetic materialCaAl LDH (FIG. 1) shows a typical layered structure with highcrystallinity, similar to those previously reported in the literaturefor LDH-like materials. For the frequently occurring carbonate form ofLDH, the basal spacing represented by the reflection at approximately10° (2?) in the XRD profile is usually equal to 0.78 nm, whereas thenitrate form has a basal spacing of 0.88 nm. The value of the basalspacing (0.86 nm) corresponds to the sum of the thickness of the[Ca₂Al(OH)₆]²⁺ layer (0.48 nm) and of the interlayer space occupied bythe anion, whose ionic diameter, in the case of nitrates, is about 0.38nm. This is in agreement with the predicted value for the basal spacing,considering a planar orientation of anions, mainly nitrates, and watermolecules within the interlayer space of LDH-like materials (FIG. 2).

The reaction of CaAl LDH with NBA, 26NS, and 2NS salts producesmaterials whose diffraction patterns are given in FIG. 1. Theintercalation between the layers of CaAl LDH samples is confirmed by anincrease in the basal spacing, the value of which represents the sum ofthe thickness of the inorganic layer (0.48 nm) and interlayer space(Table 1).

TABLE 1 RT-200° C. 200-450° C. 450-1000° C. anion size basal spacingweight loss weight loss weight loss (nm) (CPK) (nm) % % % CaAl LDH 0.380.86 9.8 17.5 16.8 CaAlNBA LDH 0.61 1.33 7.7 21.6 8.9 CaAl26NS LDH 1.261.73 12.2 18.0 25.1 CaAl2NS LDH 0.86 2.18 9.9 14.1 28.9

The results for CaAlNBA LDH and CaAl26NS LDH show that the interlayerspace available cannot accommodate NBA and 26NS molecules based on theirrespective anionic size measured on a CPK model (see Table 1).Therefore, the values obtained for the interlayer spacing could beexplained by either a grafting of the anions on the hydroxylatedinorganic layers or by tilted orientations of the molecules with respectto the double hydroxide layers. The first hypothesis is unlikely to betrue because the thermal analysis (Table 1) does not show any highthermal stability of the organic derivatives compared with the startingmaterial, CaAl LDH. For both CaAlNBA LDH and CaAl26NS, the organicmolecules can only orient themselves slightly tilted with respect to theoxide layers (FIG. 2). Conversely, the CaAl26NS LDH shows an interlayerspace concurring with the 26NS molecules lying in a perpendicularhead-to-tail bilayer arrangement towards the oxides layers (FIG. 2).

Previous intercalation results in LDH-like materials have shown thepropensity of this class of layered materials to incorporate organicanions in a perpendicular arrangement. Based on the anionic size of 26NS(Table 1), it can be assumed that there are voids between the organicmolecules themselves and between the anions and the hydroxide layers.These voids can connect to each other and can be partially occupied bywater molecules. The presence of this intercalated water was confirmedby DTG data, which indicates the presence of two distinct desorptionprocesses (physisorbed and intercalated water) from RT to 200° C. Theseobservations are comparable with previously reported results for LDH.

In the case of Disal™, the XRD pattern is shown in FIG. 3. Theintercalation of Disal™ within the interlamellar space of CaAl LDHinduced a “turbostratic” disorder as shown by the dissymmetry andenlargement of the peaks. It is also shown that upon intercalation ofDisal™, the primary peak of the starting material (CaAlLDH) was shiftedtowards low angle values, which confirms an enlargement of the spacebetween the inorganic double hydroxides CaAl layers (from d=0.86 nm forCaAl LDH to d=1.84 nm for CaAlDisal). The presence of Disal™ was alsodetected by means of FTIR analysis.

Thermal Analysis

The thermogravimetric analysis (TGA) curve of CaAl LDH is comparablewith previous data of LDH-like materials. A typical TGA/DTG(thermogravimetric analysis/derivative thermogravimetry) curve is shownin FIG. 4. The results show an initial reduction in weight between RTand 200° C. arising from physisorbed and interlayer water. A secondweight loss between 200 and 450° C. results from a concomitantdehydroxylation of the inorganic layers and a reduction of nitrates tonitrites. Beyond 450° C., a further condensation of hydroxyls anddecomposition of nitrites have been observed as reported elsewhere. Thecorresponding DTG (derivative thermogravimetry) trace shows three maineffects associated with these weight losses. Based on previous analysisand the suggested formula from the EDS analysis, Ca₂Al(OH)₆NO₃.mH₂O, thefirst weight loss corresponds to two water molecules (m=2 in theformula). The second (17.5%) and third (16.8%) weight losses, caused bya combination of structural water loss (dehydroxylation) and of nitratesdecomposition, may be described by a combination of the evaporation oftwo water molecules and the decomposition of one nitrate ion, asfollows:Ca₂Al(OH)₆NO₃→Ca₂AlO(OH)₂+2H₂O+NO

A summary of the weight loss data obtained from the thermogravimetricanalyses of CaAl LDH, CaAlNBA LDH, CaAl26NS LDH, and CaAl2NS LDH isgiven in Table 1. In the case of the controlled release formulations,the weight loss between 450 and 1000° C. is attributed to a combinationof two events: dehydroxylation of the Ca—Al hydroxide layers anddecomposition of organic molecules. A temperature treatment beyond 600°C. caused a collapse of the layered structure for all compounds and gaverise to new crystalline phases (mixed oxides, e.g., spinel forms).

Infrared Analysis

FTIR spectroscopy shows characteristic frequencies associated with thepresence of intercalated anions. FIG. 5 shows the FT-IR spectra of CaAlLDH, CaAlNBA LDH, CaAl26NS LDH, and CaAl2NS LDH. In all samples, a broadband between 3600 and 3400 cm⁻¹ represents the stretching vibrations ofthe O—H groups of the inorganic layers and the interlayer water. Anothercommon frequency for LDH-like materials is the presence of the bendingvibrations of water molecules at 1600 cm⁻¹. In the case of CaAl LDH, theabsorption centered at 1380 cm⁻¹ is assigned to the presence of nitrateanions within the structure. However, due to the broadness of this band,it may also correspond to the presence of carbonate ions, which usuallyoccurs at 1360-1370 cm⁻¹. For the controlled release formulations, thisregion is dominated by absorption bands caused by C—H stretchingvibrations in an aromatic ring. For CaAlNBA LDH, characteristic peaks ofNBA were present in the spectrum, including antisymmetric and symmetricstretchings of CO²⁻ in carboxylic acid salts (1610-1650 and 1400-1300cm⁻¹, respectively), NO₂ symmetric stretching in aromatic nitrocompounds (1360-1320 cm⁻¹), C—N stretching mode (920-830 cm⁻¹), and NO₂bending vibration in aromatic compounds (580-520 cm⁻¹). In the case ofboth CaAl2NS LDH and CaAl26NS LDH, the main characteristic peaks arequite similar and comparable with previously published data.Intercalation of the naphthalene molecules is shown qualitatively by in-and out-of-plane ring bending absorptions (640-615 and 490-465 cm⁻¹,respectively), the aromatic ring C—C single and double bonds (690 and1640-1490 cm⁻¹, respectively), and the S—O bonds of the sulfonate groupsat 1170 and 1125 cm⁻¹. The presence of all these organic bands, combinedwith XRD results, confirms a successful intercalation reaction of NBA,2NS, and 26NS within the interlayer space of CaAl LDH-like materials.

SEM/EDX

The comparison between the scanning electron micrographs (SEM) of CaAlLDH and CaAlNBA LDH (FIGS. 6 a and 6 b) shows that the former haswell-formed and regular hexagonal-shaped particles stacked on top ofeach other, a characteristic of other lamellar phases in the groups ofAFm phases and LDH-like compounds. In the case of CaAlNBA LDH, thematerial appears to be constituted of non-uniform, round-edged,hexagonal plate-like particles. The other two controlled-releaseformulations, CaAl26NS LDH and CaAl2NS LDH, show similar features.Possibly, the presence of organic molecules in the interlamellar spacehas produced a change of the superficial interaction between particlesthat influences the aggregation between particles. It is known that ifslowly precipitated, hydrotalcite-like structures give hexagonal platesthat display some edge rounding upon intercalation.

Conduction Calorimetry

Conduction calorimetry is used to monitor the hydration kinetics ofcement pastes. FIG. 7 shows an evident effect of CADisal (CalciumAluminate with superplasticizer Disal™) on the hydration kinetics of C₃S(Calcium Trisilicate). A significant difference in the hydrationkinetics was observed at both dosages of superplasticizer (Disal™) inthe solid matrix (CA) (0.06 and 0.24).

It is well known that the addition of a superplasticizer to a concretemix design induces some delay in the hydration process of cement. Inthis work, it is evident that the addition of pure Disal™ at bothdosages (0.06 and 0.24) generated a delayed hydration as shown by thecalorimetry curves in FIG. 7. When the composite additive is usedinstead of pure Disal, these curves are shifted indicating asignificantly reduced retardation effect in the hydration kinetics ofthe cement in the mix, the net effect being comparable to the controlsample (C₃S) hydration profile.

Example 3 Deintercalation of Controlled Release Formulations

In order to test the ability of the CaAl LDH to release the organicadmixture, a series of experiments were undertaken on differentcompositions and under different experimental conditions, namely pH,charge density of CaAl LDH and concentration of the organic admixtures.

The deintercalation process was tested by exposing differentformulations to a simulated concrete pore solution at room temperature.An example is described as follows:

An amount equal to 0.5 g of the formulation was added to an aqueoussolution of NaOH. The mixture was stirred at room temperature for 15,30, 60, 120 and 180 min. The resulting material was retrieved by vacuumfiltration and drying at 65° C. overnight. Two different concentrationsof NaOH were tested: 0.1 and 0.2 M.

FIGS. 8 a and 8 b depict XRD profiles of two deintercalation processesof nitrobenzoic acid (NBA). FIG. 8 a illustrates the release of NBAmolecules in a 0.1M NaOH medium. It can be seen that the main peak(d=1.33 nm) in the starting formulation CaAlNBA gradually reduced inintensity giving rise to new peaks at 2θ=11° and 29°. These peakscorrespond to the CaAl carbonated form and calcite respectively. Thecomplete release of intercalated NBA was achieved after 180 min ofstirring.

When CaAlNBA is stirred in the presence of 0.2 M NaOH aqueous solution,the deintercalation process was faster. As shown in FIG. 8 b, the majorpeak at 1.33 nm vanished completely after 15 min of stirring. Thisresult confirms the release of NBA molecules from the structure. TheCaAlNBA15 showed new peaks at around 2θ=11° and 29°, which are due tothe presence of the carbonatated form of CaAlLDH and calciterespectively. Both phases were confirmed by FTIR analysis. The processof deintercalation can be explained by an exchange reaction betweenintercalated NBA molecules and different ions present in the basicsolution such as carbonates and hydroxides. The carbonated form was verypredominant as the mixing process was done in air. It is known thatLDH-like compounds are easily contaminated by carbonates when no specialconditions (e.g. N₂ environment) are taken during the synthesis.

As the time of mixing reached 180 min, the layered character of theoriginal LDH disappeared completely and was replaced by the presence ofpeaks assigned to Al(OH)₃ and CaCO₃, as a result of calcium and aluminumion leaching.

Example 4 Testing of the Controlled Release Formulations in Cement-BasedCompositions

Different paste, mortar and concrete mixes were prepared and slump lossmeasured using the mini-slump technique (D. L. Kantro “Influence ofWater-Reducing Admixtures on Properties of Cement Pastes: A miniatureSlump Test”, Cement, Concrete, and Aggregates, Vol. 2, No. 2, Winter1980, pp. 95-102) (paste and mortar), and the standard slump method forconcrete (ASTM Test C143-90a “Standard Test Method for Slump ofHydraulic Cement Concrete”).

In this example, the organic admixture was a sulphonated naphthaleneformaldehyde-based superplasticizer, called Disal™. The controlledrelease formulation (CADisal) was synthesized in a manner as describedin Example 1.

The effectiveness of Disal™ alone in controlling the slump-loss versustime characteristic was compared to that of the controlled releaseformulation CADisal. A normal Portland Type 10 cement was used unlessotherwise indicated. A standard Ottawa Illinois ASTM C778 grade sand wasused for mortar and concrete mixes. Aggregates passing a 10 mm sievewere added to the concrete mixes.

4A: Cement Pastes

Cement paste control specimens were prepared and mini slump measurementstaken. The following test sequence was used.

Pastes with a water-cement (w/c) ratio of 0.50 were produced having aDisal™ dosage of 0.3% by mass of cement. A mini-slump cone (d(top)=19.1mm; d(bottom)=38.1 mm; h=57.2 mm) was used for slump measurements. Thefollowing mixing sequence (2 min stirring; 3 min standing; 2 minstirring) was employed. Slump-loss vs. time curves with 1 min mixingafter each interval of storage were produced.

Cement paste specimens containing CADisal were also produced by addingthe CADisal to cement to form a mixture, and then adding the mixture towater with mixing. The same procedure as described above was used toproduce slump-loss vs. time curves for CADisal. The dosage of CADisalwas 2.4% by mass of cement and the w/c was 0.5.

FIG. 9 is a plot of the slump loss of cement pastes produced with Disal™alone and with the controlled release formulation (CADisal) as afunction of mixing time at room temperature. As shown on the plot, theslump loss of cement paste produced with pure Disal™ and with thecontrolled release formulation with respect to the elapsed timeindicates that both samples have the same trend (steep slump loss) up to30 min for CADisal and up to 100 min for pure Disal™. It is important tonotice that when CADisal is used (especially for 2.4% Disal™), themagnitude of slump loss is considerably slower (due to controlledrelease) than when pure Disal™ is added to the mix. Overall, the plotindicates that the controlled release formulation (2.4% Disal™) providesa longer time for the superplasticizer to keep cement workability at areasonable level after mixing.

After 30 min of mixing, the workability of the mix is almost steady(plateau) up to 210 min. After that, the mix starts to lose itsconsistency. This is a very important factor in concrete-making. Indeed,prolonged mixing in a truck mixer induces acceleration of stiffening ofconcrete and consequently an increased rate of the slump loss. The timethat elapses in the course of mixing, delivering, placing, compacting,and finishing operations is considered to be a key parameter for slumploss. The direct consequence of this is difficulty in handling andplacing, reduction of ultimate strength and decreased durability. Thecontrolled release formulation of the present invention helps alleviatethis difficulty.

4B: Mortars

Mortar control specimens were prepared and mini slump measurementstaken. The following test sequence was used.

Mortars with a water-cement (w/c) ratio of 0.59 and a cement/sand (c/s)ratio of 1:2.75 (by mass) were produced. A modified cone for slump lossmeasurements (d(top)=37.5 mm), d(b)=75 mm; h=112.5 mm) was used. Disal™dosage used was 0.3% by mass of cement. The procedure described abovefor the pastes was used for the mortar mixes.

Mortar specimens containing CADisal were also produced. The sameprocedure as described above to produce slump-loss vs. time curves wasused. Three dosages of CADisal, 2.4%, 3.6% and 4.8% by mass of cement,were used and the w/c was 0.59.

The slump loss test results performed on mortar mixes containing pureDisal™ (control), and 2.4%, 3.6% and 4.8% of CADisal formulation areshown in FIG. 10. All the mortar samples lost slump with time. Clearly,the incremental loss in slump was more pronounced in the case of thecontrol sample. While some differences were noted, the basic trends weresimilar for all samples including the controlled release formulationCADisal. They all showed a steady loss in slump with time. Increaseddosage of CADisal induced an increased initial slump level.Nevertheless, the sample having 3.6% CADisal exhibited a substantialimprovement in slump retention with time with an initial lump levelcomparable to the control sample. After about one hour, the measuredvalue for the slump was 8.7 and 9.7 mm for the control and 3.6% CADisalsamples, respectively. The control showed significant stiffening at 110min while all the other mixes kept their workability until about 200min.

4C: Concrete

Concrete control specimens were prepared and standard slump measurementstaken. The following test sequence was used.

Concrete with a w/c of 0.59 and cement:sand:aggregate ratio of 1:2:3.2(by mass) was used. Disal™ dosage was 0.3% by mass of cement. A standardslump cone (measurements of both height change and change in base area)was used. The test procedure described above for the paste and mortarsamples was followed.

Concrete containing CADisal was also produced. The same procedure asdescribed above to produce slump-loss vs. time curves was used. Thedosages of CADisal were 2.4% and 3.6% by mass of cement and the w/c was0.59.

Curves depicting slump area versus time for different concrete mixes aregiven in FIG. 11. The control concrete mix with pure Disal™ shows acontinuous loss in slump throughout the test with a more pronounced lossin the initial 50 min. It was observed that towards the end of the test,the mixture was starting to become stiff. In the cases of the concretemix containing 2.4% and 3.6% CaAlDisal, initial slump values were higherthan in the case of pure Disal. This behaviour may be due to acombination effect of both the CaLDH and the presence of Disal. Bothsamples at 2.4% and 3.6% showed a better control of slump loss withtime, with a continuous decrease for 2.4% and a gradual decrease with aplateau for 3.6%. This trend is basically identical to what was observedwith mortar mixes: a gradual loss up to 50 min, a plateau, and then acontinuous decrease in slump. This behaviour was also clearlydemonstrated with paste samples as they showed a better control for therelease of the admixture compared to the control mix. Overall,regardless of CaDisal dosage, the effect of the controlled releaseformulation remains essentially the same for both samples.

LDH-based controlled release formulations of the present invention areeffective for organic admixtures used in cement-based compositions, forexample, accelerators (e.g. para and meta nitrobenzoic), retarders (e.g.ortho, para, and meta aminobenzoic acid), superplastcizers (e.g.naphthalene-2-sulfonate). The results have confirmed the effectivenessof the present formulations in controlling the workability ofcement-based compositions, especially in respect of slump losscharacteristics of pastes, mortar and concretes.

Other advantages that are inherent to the structure are obvious to oneskilled in the art. The embodiments are described herein illustrativelyand are not meant to limit the scope of the invention as claimed.Variations of the foregoing embodiments will be evident to a person ofordinary skill and are intended by the inventor to be encompassed by thefollowing claims.

1. A method of controlling release of an admixture in a cement-basedcomposition comprising first intercalating the admixture into a layereddouble hydroxide or mixture thereof to form a controlled releaseformulation and then adding the controlled release formulation to acement-based material in an amount of from 0.2% to 10%, based on mass ofthe cement-based material.
 2. The method of claim 1, wherein thecement-based material comprises cement, mortar or concrete.
 3. Themethod of claim 1 wherein the controlled release formulation is added tothe cement-based material in an amount of from 1% to 5%, based on massof the cement-based material.
 4. The method of claim 1 wherein thecement-based material comprises Portland cement.
 5. The method of claim1, wherein the layered double hydroxide has a formula (I):[M²⁺ _(1-x)M³⁺ _(x)(OH)₂]^(x+)[A^(n−) _(x/n) .mH₂O]^(x−)  (I) where M²⁺is a divalent metal cation, M³⁺ is a trivalent metal cation, A^(n−) isan inorganic anion, x is a number from 0 to 1 but not 0, n is a number 1or greater, and m is a number 0 or greater.
 6. The method of claim 1,wherein M²⁺ is Ni²⁺, Zn²⁺, Mn²⁺, Ca²⁺ or a mixture thereof, M³⁺ is Al³⁺,Ga³⁺, Fe³⁺, Cr³⁺ or a mixture thereof, and A^(n−) is NO₃ ⁻, Cl⁻, CO₃ ²⁻,SO₄ ²⁻ or a mixture thereof.
 7. The method of claim 5, wherein M²⁺ isCa²⁺ and M³⁺ is Al³⁺.
 8. The method of claim 5, wherein x is from 0.25to 0.33.
 9. The method of claim 5, wherein n is from 1 to
 4. 10. Themethod of claim 5, wherein x is 0.25 and the admixture is present in thecontrolled release formulation in a concentration of 0.5 M or 0.7 M. 11.The method of claim 5, wherein the admixture is present in thecontrolled release formulation in a concentration of 0.05-1 M.
 12. Themethod of claim 5, wherein the admixture is present in the controlledrelease formulation in a concentration of 0.5-0.7 M.
 13. The method ofclaim 5, wherein the admixture comprises an accelerator for reducing settime, a retarder for delaying set time, a superplasticizer, anair-entraining agent for freeze-thaw resistance, a corrosion inhibitor,an expansive admixture for minimizing shrinkage, a shrinkage reducingadmixture, a water repelling admixture, a water reducer, analkali-aggregate reaction inhibitor, or a mixture thereof.
 14. Themethod of claim 5, wherein the layered inorganic material comprises alayered double hydroxide or mixture thereof, and wherein the admixturecomprises an accelerator for reducing set time, a retarder for delayingset time, a superplasticizer, an air-entraining agent for freeze-thawresistance, a corrosion inhibitor, an expansive admixture for minimizingshrinkage, a shrinkage reducing admixture, a water repelling admixture,a water reducer, an alkali-aggregate reaction inhibitor, or a mixturethereof.
 15. The method of claim 14, wherein the admixture comprises asuperplasticizer.
 16. The method of claim 14, wherein the admixturecomprises a sulphonated naphthalene formaldehyde-based superplasticizer.17. The method of claim 14, wherein the admixture comprises analkali-aggregate reaction inhibitor comprising a lithium-based salt.